Can you cake a Challenge ?

As a youngster Mad Ade had a job working as an errand boy.
One day he was given the task of collecting boxes of cakes for the Summer Fair.
He collected boxes from various people in his local village and
each box was labelled in Roman Numerals with the number of cakes in the box.
By the time Mad Ade had collected the last box, he was quite hungry,
and really need to eat at least one cake.
Luckily the last box was marked with XI, meaning there were 11 cakes in it.
He had the brain wave of turning the box around and underlining the number to give the impression there were IX, that is 9 cakes.
However, after eating the 2 cakes, (predictably) Mad Ade was still hungry.
How did the young Mad Ade change the number shown on the box again and
eat even more cakes, without any one knowing ?